var mergTwoLists=function(l1,l2){
    if(l1===null){
        return l2
    }else if(l2===null){
        return l1
    }else if(l1.val<l2.val){
        l1.next=mergTwoLists(l1.next,l2)
        return l1
    }else{
        l2.next=mergTwoLists(l1,l2.next)
        return l2
    }
}